1. Field of the Invention
The present invention relates to an optical transmitting apparatus, an optical receiving apparatus, and an optical transmitting-receiving system. For example, the present invention is applicable to a device or a system for optical interconnection, in which data are sent and received between a plurality of boards and devices by the parallel synchronous transmission of optical signals.
2. Description of the Related Art
In a parallel computer or an electronic data interchanging system, an optical interconnecting method in which data are sent and received by the parallel synchronous transmission of optical signals is applied to the transmissions between a plurality of boards or devices (cabinets). The application is examined and studied as described in A. Takai et al. "(The Current Condition of) Optical Interconnects" (Shingakugiho, LQE96-150, February 1997).
According to the optical interconnecting method, in general, at a transmitter side, each of a plurality of transmission data in a synchronous state makes an electrical-to-optical conversion by a laser diode corresponding to each of the transmission data, and the data is sent to an optical fiber corresponding to each of the transmission data among a plurality of optical fibers mounted in tape shapes or coaxial shapes. At a receiver side, each of the transmission data (optical signals) transmitted via the optical fiber makes an optical-to-electrical conversion by corresponding photodiodes, and thereafter, the size of each transmission data converted into an electric signal is compared to a fixed identification level. A code (logical level) of each transmission data is thereby determined.
Also, in a case in which the code (logical level) of each received data is determined at the receiver side by using the fixed identification level, the higher logical levels in the received data are dispersed due to temperature characteristics of active elements such as laser diodes and photodiodes and due to the dispersion of optical power transmitted via the optical fiber. With the aforementioned in view, the fixed identification level must be lowered. Accordingly, voltages of lower logical levels in the received data, which are to be sufficiently smaller than the fixed identification level, must be set as small as possible. Thus, conventionally, a direct current bias is not applied to the laser diode (e.g., 0 mA). As a result, a pattern effect that is characteristic to the digital modulation of the laser diode becomes apparent. Namely, in a case in which the transmission data having a higher logical level is supplied to the laser diode and oscillates again, the rise time of oscillation of the laser diode changes depending on the patterns of the logical levels of the past transmission data. More concretely, in the patterns of the logical levels of the past transmission data, in a case in which there are many higher logical levels, the rise of oscillation of the laser diode occurs quickly, and in a case in which there are few higher logical levels, the rise of oscillation of the laser diode occurs rather slowly.
In a plurality of transmission data which is subjected to synchronous transmission, the changing patterns of the logical levels are often different from each other. As a result, due to the difference of the rise times of oscillation among each of a plurality of laser diodes that executes an electrical-to-optical conversion of each of the data, a skew in which phases of each of the regenerated data are not uniform occurs when regenerating the data at the receiver side.
Further, in the method in which the code (logical level) of each received data is determined at the receiver side by using the fixed identification level, as shown in FIG. 2, the skew is changed due to the levels of the plurality of received data, which have made an optical-to-electrical conversion (the larger the difference in the levels, the larger the skew to some extent). In a case in which the levels of received data are changed by temperature or the like, the skew may also be changed.
A drawback was described above in consideration of the synchronous parallel transmission of optical signals. Even when a series of transmission data is optically transmitted in accordance with the conventional method in which the fixed identification level is used for determining the code, there are drawbacks due to the pattern effect and due to the variation in the received levels.
For example, in a case in which a series of transmission data is optically transmitted, a clock signal for regeneration is formed from the regenerated data by using a PLL circuit or the like. When there is a time-axis variation (jittering) due to the pattern effect and due to the variation in the received levels, it is greatly possible that a good regeneration clock is not created.
Accordingly, an optical transmitting apparatus, an optical receiving apparatus, and an optical transmitting-receiving system, in which the time-axis variation in regenerated data can be minimized, are desired. Further, if a plurality of transmission data is synchronously transmitted, an optical transmitting apparatus, an optical receiving apparatus, and an optical transmitting-receiving system, in which the phases of each received data are arranged as uniform as possible, are desired.